BBC Sky at Night - UK (2021-08)

August 2021 BBC Sky at Night Magazine 69

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northern latitudes are positive, southern negative.

Longitude determination is done by timing.

Visualise an imaginary line running from the planet’s

north pole to its south pole, and this imaginary line

is known as the planet’s Central Meridian (CM). As

the rotation period of each planet’s atmosphere is

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longitude) as the longitude at the CM at an agreed,

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longitude increases until it’s reset as the chronological

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A complication occurs due to the equatorial

regions rotating faster than the rest of the

atmosphere. This is addressed by dividing regions

into two longitude zones: System I is used for the

equatorial region between latitudes +10 and –10,

while System II covers everything else. However,

a further longitude system also exists, known as

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planet’s magnetosphere to rotate. Although System

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amateur astronomers normally use System I and II.

Keeping track

As Jupiter and Saturn rotate quickly, over several

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feature passing the CM. For small features, accurately

recording the time (in Universal Time) when this

happens allows you to determine the longitude of

that feature. If you make longitude measurements

of the same feature over a long period of time,

ranging over months or even years, you will be able

Filters can be used to enhance both visual

and imaging sessions. The Wratten (W)

scheme is commonly employed for visual

planetary observation, with filters

normally identified by ‘W’, followed by a

number and sometimes a letter.

Visually, a light blue filter (W38A or

W80A) enhances contrast within brighter

zones and draws out detail in atmospheric

clouds. Light green (W57) and blue (W47)

helps to darken brown-hued belts. A W 57

light green or a magenta (W30) filter also

helps highlight Saturn's rings. Using a

W30 filter, meanwhile, will brighten white

ovals in Jupiter’s atmosphere.

When imaging, mono camera setups

work best for filter work. Full colour

images are made by imaging through red,

green and blue imaging filters and

combining the results. Infrared (IR) pass

filters produce high-contrast results,

great for showing definition. Some colour

cameras have good IR sensitivity and can

be used in combination with such filters.

Speciality filters, such as those centred

on the methane absorption band (CH4),

produce valuable results for Jupiter.

Centred on a wavelength of 890nm and

with bandpass windows typically less

than 15nm across, light is at a premium

with this filter making it more suitable for

larger instruments. A CH4 filter brightens

high-altitude atmospheric features.

Specific colours and wavelengths will enhance your view of Jupiter and Saturn

Using filters on the gas giants

Þ Filters can be used to enhance features on Jupiter and Saturn for both visual and imaging setups

Þ The coordinate

system on Jupiter

uses a similar

latitude system to

Earth, but longitude

is determined from

a timed Central

Meridian (CM) >

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